This invention relates generally to solar cell assemblies and second surface mirrors which are especially useful in space, and more particularly to a method for testing such solar cell assemblies and second surface mirrors to identify those that have a susceptibility to ultraviolet degradation.
Solar panels are conventionally used as a source of electrical power for spacecraft such as satellites. The solar panels typically used for spacecraft include a substrate and a plurality of individual photovoltaic solar cells which are secured to a face surface of the substrate. The individual solar cells are electrically connected together to form a series-parallel solar cell array which, when oriented properly toward the sun, converts solar energy into electrical energy. A cover glass, typically made of a borosilicate material, covers the individual solar cells and together with the cells forms a solar cell assembly.
The efficiency of a solar cell is directly related to the amount of useful light which is absorbed by the solar cell. Only a portion of the light striking the top surface of a solar cell-is useful to the cell. Another portion of the light striking the cell is nonuseful, i.e., the light has wavelengths outside the range useful to the cell; and yet another portion of light is reflected by the solar cell. To reduce the problem of light reflection, solar cells employ an antireflective (low reflectance) coating positioned on the surface of the solar cell through which light enters. To reduce solar heating effects, an alternative coating design provides antireflective properties over only the useful range of wavelengths while also highly reflecting non useful wavelengths.
One of the most important consideration for solar assemblies and panels used on spacecraft is efficiency. If a solar panel degrades in space, it is difficult, if not impossible, to correct or compensate for the resulting loss of electrical power with the result that the useful life of the entire spacecraft is often prematurely ended.
Second surface mirrors are fused silica or borosilicate glass with a silver coating on the back side that are used to radiate thermal energy from heat producing components, and reflect incident solar radiation. Solar energy passes through the fused silica and is reflected from the silver surface to prevent excessive heating when exposed to the sun. Efficiency can be seriously degraded by very small amounts of surface contamination that absorb energy on the mirrors surface. Layers of contaminants as little as 300 Angstroms thick will darken when exposed to the space environment and result in a serious increase in the solar absorptance of the second surface mirrors. This will result in increased heating of the mirrors when exposed to sunlight and potential degradation of the spacecraft performance due to excessive heating.
In the deployed configuration, the solar panels are subjected to substantial thermal stresses; the solar cells and the front surfaces of the substrates and cover glasses are subjected to the intense heat of the sun while the back surfaces of the substrates are subjected to the extreme cold of outer space. Furthermore, the cover glasses may be susceptible to degradation (visible darkening) upon exposure to ultraviolet radiation and other radiation in the space environment. Some degradation is expected to lead to a nominal, end-of-life, loss in performance. Abnormal degradation however may be caused by deleterious defects such as impurities and/or contamination sites present in the cover glass and coatings. These defects can absorb radiation at a particular ultraviolet wavelength or wavelengths and can result in visible darkening. This darkening of the cover glass results in less useful light transmitting to the solar cell material, which in turn directly lowers the efficiency and power generated by the solar cell. The darkening may also result in an undesirable increase of the temperature of the assembly arising from the increased solar absorption by the darkened cover glass.
Conventional quality control methods for inspecting solar cell cover glasses examine properties such as trace element levels in the glass substrate and UV reflectance coatings, multilayer coating thickness, and optical quality. These measurements do not necessarily correlate with the propensity of a cover glass to darken on-orbit. In addition, inspectors are not able to inspect cells once the cells are mounted to a solar array. The current inspection methods are susceptible to passing solar cell assemblies that initially meet quality specifications but later degrade upon ultraviolet-induced darkening. In addition, the current inspection methods will not detect contaminant layers which accumulate after the solar cell has been integrated into the array. It thus is desirable to provide a test method that identifies the susceptibility of the solar cell assemblies to on-orbit darkening and degradation and can be practiced on a completed solar array.
Currently, there is no reliable method available to directly measure the amount of surface contamination that is present on second surface mirrors once they are installed on a spacecraft other than visual inspection. The current method used to verify the cleanliness of a second surface mirror includes repeated cleanings and careful protection of the mirrors from contamination sources. This can be costly and requires carefully controlled work areas and elaborate environmental enclosures. In addition, excessive contamination that is left on second surface mirrors and that is not detected has proven to result in overheating and seriously degraded performance. If the cleanliness could be verified prior to launch, the likelihood of degradation can be substantially reduced. As such, better knowledge of the amount of on-orbit degradation would allow the use of less design margin resulting in less radiator area and heater power required for proper spacecraft operation. What is needed therefore is a method that detects cleanliness of a second surface mirror and that can be used close to the launch time of the spacecraft.
The aforementioned need in the prior art is met by the present invention which provides a method for determining the susceptibility of solar cell cover glass to degradation as well as determining the contamination levels on second surface mirrors. The method comprises the steps of (1) illuminating the solar cell cover glass or second surface mirror with a benign exposure to ultraviolet light at a preselected illumination angle where the solar cell cover glass or the silver layer of the second surface mirror reflects a portion of the ultraviolet light; (2) measuring the reflected portion of the ultraviolet light; and, (3) characterizing the propensity of the cover glass to degradation or characterizing the amount of surface contamination on the second surface mirror as a function of the reflectance.